It is well known in the art that there is a need for electronic countermeasure (ECM) equipment to counter a Doppler type tracking radar. Doppler radars are widely employed as discrimination between stationary and moving targets is possible by the use of the Doppler principle. Doppler radar systems have been employed which are either continuous or pulsed Doppler radar systems. In its simplest application a CW transmitter is used and the return energy is detected by mixing it with a portion of the transmitted power. Fixed targets produce a constant voltage whereas a moving target produces an alternating voltage at the Doppler frequency difference between the transmit and receive signals. This Doppler frequency is extremely well known. Each cycle of Doppler frequency normally corresponds to a target radial motion of one half of the transmitted wavelength. Thus, a target moving with a radial velocity of 300 miles per hour or 444 feet per second will move about 880 half waves per second at 1,000 mHz resulting in a Doppler frequency of about 880 Hz. As indicated, Doppler radar systems are widely employed. Such Doppler systems can be utilized to track a target and hence to provide a display indicative of the target's path. The signals from the radar can be used for weapon control, and hence countermeasures are needed to confuse the tracking radar or to jam the radar. Specifically, in countering such Doppler type tracking radars, it is well known that there is a need for a Doppler shift generator apparatus to provide velocity deception. Certain presently developed Doppler shift generators use a repeater which utilizes a frequency translation or serrodyne apparatus. The use of such repeaters in electronic countermeasure equipment for countering Doppler type tracking radars is difficult based on the fact that Doppler radars have a high degree of frequency accuracy and measurement resolution. Hence, the use of signal generation from a repcater or ECM device which attempts to set an RF oscillator onto precisely the same frequency as the signal received from such radars, or onto a frequency which is close thereto is extremely difficult.
It is well known to those of ordinary skill in the art that electronic countermeasure (ECM) equipment which can delay a received signal in time for the purpose of range falsification and which can shift the frequency of the received signal for the purpose of velocity deception requires some sort of signal storage. RF memory devices that have been used in the past to satisfy this need include digital RF memory devices (DRFM) and analog RF memory devices (ARFM). For example, a typical ARFM is comprised of a high gain traveling wave tube (TWT) and delayed feedback to provide serrodyne, or frequency translation by mixing. However, these RF memory devices suffer from several drawbacks, such as, low reliability, high noise, large size, large weight, spurious and harmonically related RF output signals, and narrow RF bandwidth. Specifically, DRFM suffer from large size and low performance because of the need to perform multiple analog-to-digital conversions which are followed by digital-to-analog conversions. Such conversions lead to spurious spectral contributions, poor phase coherence, and bandlimited operation. Further, specifically, ARFM using traveling wave tubes (TWT) type analog memory loops and serrodyne translation are very noisy, lack phase coherence and generate many spurious signals. Further, ARFM suffer from the additional disadvantage that long term storage of the received signal is not possible. As a result, ARFM does not permit one to fabricate electronic countermeasure equipment that can selectively delay a transmitted signal for the purpose of completely covering a received radar pulse that is reflected from the countermeasure platform being protected and for the purpose of generating false range targets.
As a result, there is a need in the art for a Doppler shift generator for use in electronic countermeasure equipment which is smaller in size, higher in performance, has fewer spurious spectral contributions, better phase coherence, and a wider band of operation than existing apparatus fabricated using DRFM or ARFM. Further, there is a need for such apparatus which can provide long term storage of the received signal so that electronic countermeasure equipment can selectively delay a transmitted radar signal with respect to a received signal for the purpose of completely covering a received radar pulse that is reflected from the countermeasure platform being protected and for the purpose of generating false range targets.